Non-volatile memories such as memristors and Phase Change Memory (“PCM”) have emerged as promising and scalable alternatives to currently prevalent memory technologies like Dynamic Random Access Memory (“DRAM”) and Flash memory. In addition to a fundamentally different approach to storing data that leads to a higher memory density, a lower cost per bit, and a larger capacity than DRAM and Flash, these emerging non-volatile memories support multi-level cell (“MLC”) technology, which allows each memory cell to store two or more bits (DRAM, in contrast, can only store one bit per cell). The potential to operate at a lower power further adds to memristors' and PCM's competitiveness as scalable DRAM alternatives.
In particular, PCM is an emerging memory technology that stores data by varying the electrical resistance of a material known as chalcogenide. By applying heat, and then allowing it to cool at different rates, chalcogenide can be manipulated to settle between an amorphous (quickly quenched) high resistance state (e.g., a logic low or zero) and a crystalline (slowly cooled) low resistance state (e.g., a logic high or one). PCM is non-volatile, as the state of chalcogenide is retained in the absence of electrical power. The large resistance difference by three orders of magnitude) between the amorphous and crystalline states of a PCM cell enables MLC technology to be implemented in PCM cells. This is achieved by partitioning the large resistance difference into four distinct regions, each representing a two-bit value of “11”, “10”, “01”, and “00”. More than one bit can be stored in a cell by controlling its resistance precisely to lie within one of these resistance regions.
Supporting MCL in PCM, however, incurs higher access latency and energy. MLC requires the cell resistance to be controlled precisely to lie within a narrower range, which necessitates iterative writing and reading techniques with multiple sensing iterations that lead to higher read latency and energy as well as higher write latency and energy.